1. Field of the Invention
The present invention relates to a ringing detector which receives a digital signal and determines whether or not ringing has been generated in the received digital signal, and a filter using the ringing detector.
2. Description of the Related Art
In order to reduce a required amount of data for recording or transmitting an image signal, it is effective to perform decimation processing in a horizontal direction or in a vertical direction of a screen with respect to the sample values of the image signal. In a conventional decimation processor, a filter is used for limiting the bandwidth to a Nyquist frequency or lower before the decimation processing is performed.
On the other hand, if a filter for limiting the bandwidth to a Nyquist frequency or lower is used for performing an interpolation, all the frequency components having a frequency equal to or lower than the Nyquist frequency are completely restored in contrast with the former case.
In this case, in order to transmit as much of the information of the original signal as possible, it is necessary to reduce the attenuation amount of the signal in the pass band by inclining the slope representing the cutoff characteristics of the filter as steeply as possible.
However, as the slope representing the cutoff characteristics of the filter is inclined more steeply by increasing the number of taps of the filter, a deterioration called "ringing" is generated more remarkably in the quality of the resulting image. To the contrary, as the slope representing the cutoff characteristics is inclined more gently by decreasing the number of taps of the filter, the attenuation amount of the signal in the pass band is increased, so that the information amount of the signal is decreased.
Therefore, according to a conventional technology, a filter has been designed so as to have moderately steep cutoff characteristics based on the attenuation amount of the input signal and the generation amount of the ringing. On the other hand, according to another conventional technology, the ringing of the signal passed through a low-pass filter is detected, whereby the cutoff characteristics of the filter are varied in accordance with the detected ringing.
For example, a method in which a signal processed by a low-pass filter is passed through a high-pass filter and the existence of the ringing is detected based on the output signal of the high-pass filter is disclosed in Japanese Laid-Open Patent Publication No. 1-276880. In accordance with this method, it is highly possible that the filter is erroneously operated by the high frequency components included in the input signal itself, or that the high frequency components of the input signal are erroneously detected as the ringing generated by the filtering processing. As a result, the high frequency components included in the input signal are sometimes lost.
On the other hand, a method for detecting the existence of the ringing based on a ratio of the maximum amplitude of the pixel value of a pixel existing in the vicinity of a remarked pixel (or a currently processed pixel) to the maximum amplitude of the pixel value of a pixel existing in the ringing detection region is disclosed in Japanese Laid-Open Patent Publication No. 5-56306.
In the above-described two kinds of methods, an edge portion (i.e., a boundary between a plurality of regions or a line dividing a plurality of regions) of an image is determined as having a ringing, so that the edge portion becomes blurred or thickens. Furthermore, in both of the two methods, the existence of the ringing is detected for a signal subjected to a decimation or an interpolation by using a high-pass filter and then processing for suppressing the ringing is performed. A signal which has passed through a high-pass filter for detecting a ringing reflects the characteristics of the filter, more specifically the impulse response thereof. Thus, in accordance with the above-described conventional methods, it is impossible to correctly determine whether the detected components are the high frequency components included in the input signal itself or the ringing generated by the filtering.